Before a facsimile image is transmitted from an originating facsimile machine to a terminating facsimile machine, the originating and terminating facsimile machines send messages between the machines to negotiate transmitting and receiving modes for transmitting facsimile data. Such transmitting and receiving modes specify a set of parameters that are agreed upon prior to transmitting facsimile data. These parameters may include a data bit rate, a page size, the number of pixels per line, the number of lines per page, and whether or not any fill characters are added to a line so that the terminating facsimile machine has time to print each line.
The negotiations between two facsimile machines takes place in what is known as "phase B" of the facsimile session. During this phase, messages referred to as T.30 HDLC messages are sent between facsimile machines in accordance with the T.30 procedures for Document Facsimile Transmission in the General Switched Telephone Network specification, which is promulgated by the International Telecommunication Union (ITU). Some of these messages passed during the negotiation phase may be classified into two groups: a first group which may be referred to as "transmission mode offer messages," and a second group which may be referred to as "transmission mode acceptance messages." As used herein, a transmission mode offer message is an offer from one facsimile machine to another to operate in one or one of a set of transmitting and receiving modes. Because this message merely offers to operate in one, or one of a set of transmitting modes, it is up to the offer-receiving facsimile machine to accept a transmission mode by responding with a transmission mode acceptance message.
Transmission mode acceptance messages are messages that make a decision, or give tentative or final approval for a particular transmitting mode. These acceptance messages are sent from an acceptance-transmitting facsimile machine to an acceptance-receiving facsimile machine. It may be said that in a successful negotiation, transmission mode offer messages are always followed by a transmission mode acceptance message. And, at different points in the negotiation, the acceptance-transmitting and acceptance-receiving facsimile machines may be either the originating or terminating facsimile machine. That is, at some point in the negotiation the originating facsimile machine may be the acceptance-transmitting facsimile machine, and at another point, the originating facsimile machine may be the acceptance-receiving facsimile machine, with the complimentary relationship being true for the terminating facsimile machine as well.
An example of a transmission mode offer message is a Digital Identification Signal (DIS) message, which is a message from a terminating facsimile machine that discloses the terminating machine's capabilities in the form of an offer to receive a facsimile transmission that is transmitted in one of a set of offered transmission modes. Note that this offer message does not necessarily select or decide, either tentatively or finally, upon a transmitting or receiving mode. Thus, the transmission mode offer message may, f or example, disclose that the terminating facsimile machine is capable of operating in modes defined by specifications V.17, V.29, V.33, and V.27ter, which are specifications promulgated by ITU.
An example of a transmission mode acceptance message sent in response to an offer message is a Digital Command Signal (DCS) message. This message normally accepts one of the transmission modes that was offered in the DIS message. This acceptance may be considered a tentative or conditional acceptance that may later be finally accepted following a successful transmission of a training sequence.
Because the negotiation is a back-and-forth process between two facsimile machines, other transmission mode offer and acceptance messages may follow the DCS message. For example, a Training Check (TCF) message may be considered an offer to test a communication link by transmitting a known training sequence at a rate, or in a mode, that was conditionally accepted by the DCS message. This offer to test the link by sending, for example, a series of zeros transmitted at the conditionally accepted rate may be subsequently accepted by a Confirmation to Receive (CFR) message, which is a transmission mode acceptance message that tells the originating facsimile machine that the training sequence has been successfully received and that the communication link between the two machines will probably support the accepted mode of facsimile data transmission.
As described above, the originating and terminating facsimile machines may progress through two or more rounds of offer and acceptance messages to finally arrive at the agreed upon transmitting and receiving modes for the facsimile session.
When an originating and terminating facsimile machine need to transmit facsimile data via a medium having a variable data transmission time, the negotiation of transmitting and receiving modes becomes important to the success of sending a facsimile image. An example of a medium having a variable data transmission time is a wireless medium, which may, for example, be used to provide wireless local loop telephone service to businesses and homes. The reason this wireless communications system may be considered a medium having a variable data transmission time is that the wirelessly transmitted data may be influenced by noise or interference from other users, and thus may need to be retransmitted to correct errors. This retransmission takes time and may introduce delay in the overall communications link between the originating and terminating facsimile machines. The internet or other similar network is another example of a communication medium having a variable data transmission time. Such mediums do not have a low, fixed time of data propagation. Other examples include a medium that allows random quantities of data with variable periods of idle or null characters to be synchronously transmitted.
In a wireless local loop system, telephones, facsimile machines, and other terminal equipment at the customer's site may be connected to a device frequently referred to as a fixed wireless terminal (FWT). The fixed wireless terminal provides basic telephone local loop service through a wireless or radio link. In many service areas, telephone service may be provided wirelessly at a fraction of the cost of traditional wireline infrastructure. Other benefits of wireless local loop systems are rapid deployment time, the ability to cover a large area, high capacity, and lower operating and maintenance costs.
Many wireless local loop systems are implemented with a digital air interface between the fixed wireless terminal and a base station transceiver. This means that signals from a facsimile machine connected to the fixed wireless terminal must be converted from an analog signal to a digital signal before transmission over the air. Similarly, digital signals received from the base transceiver must be converted to analog signals in the FWT before they are sent to a connected facsimile machine.
The digital air interface of the wireless system typically provides for retransmitting the data if an error is detected while receiving data. Thus, the digital air interface uses a flow-controlled protocol which may delay a stream of data while erroneously received data is being retransmitted. In contrast to this FWT wireless interface, the facsimile machine may not use flow control during the transmission of facsimile image data. This may cause a problem when the data transmission rate of the originating facsimile machine is close to the data transmission rate of the air interface. If the air interface efficiency drops due to retransmissions because of noise or other interference, the originating facsimile machine may complete the transmission of facsimile image data long before the fixed wireless terminal has transmitted the same facsimile image data over the air and to the terminating facsimile machine.
To further complicate the problem, the originating facsimile machine expects a timely reply from the terminating facsimile machine after a page of facsimile data is transmitted, wherein such a reply confirms the reception of the facsimile image data. For example, if a Message Confirmation (MCF) is not received by the originating facsimile machine within a predetermined period of time, the originating facsimile machine will disconnect the call, and further pages will not be transmitted, nor will the facsimile session end with a proper confirmation. The resulting failure to send a multipage facsimile image is a major problem with current methods of transmitting facsimile data via a communication link having a media with a variable time of data transmission.
In the prior art, it has been suggested that a fixed wireless terminal connected to a terminating facsimile machine change a transmission mode offer message from the terminating machine so that it appears to the originating facsimile machine that the terminating facisimile machine can only receive data in a mode having a slower rate than its actual maximum rate. By changing the transmission mode offer message, the fixed wireless terminal connected to the terminating facsimile machine influences the negotiation process so that the originating facsimile machine transmits at a slower rate than the maximum rate of, perhaps, both the originating and terminating facsimile machines. The purpose of this influence over the negotiation is to reduce the data rate of the entire communications link that supports the facsimile session in hope that the rate is slow enough for the air interface, and that errors in the air interface are less likely in a slower facsimile session. It is important that higher-rate facsimile machines do not independently negotiate a facsimile session having data speed that exceeds the effective throughput of the wireless communications link.
While the discussion above relates to a wireless communication link in a wireless local loop communications system, other communications media having a variable data transmission time may have similar problems supporting a facsimile data transmission session. Such other communications media may also retransmit erroneously received data packets, thus causing data transmission time over the media to be variable and to lag behind the capabilities of the facsimile machines.
While slowing down the communications link for the facsimile session may prevent the facsimile machines from negotiating a data rate higher than the link will support, the facsimile session over this slower link is less efficient than it could be. Therefore, there is a need for an improved method and system for negotiating transmitting and receiving modes for transmitting facsimile data via a medium having a variable data transmission time, wherein the improved method and system increases the efficiency of facsimile data transmission while maintaining the ability to correct errors that may occur in the data transmission medium, and while strictly adhering to the timing requirements of control messages transferred in a facsimile session.